Numerous processes use radial flow reactors to effect the contacting of particulate matter with a gaseous stream. These processes include hydrocarbon conversion, adsorption, and exhaust gas treatment. These reactors contain a vertically extending annular bed of particles through which the gases flow radially in an inward or outward direction. The annular bed is formed by an outer screen element located along the outer diameter of the particle bed and an inner screen element located along the inner diameter of the particle bed. The outer screen element often consists of a series of closed conduits having an oblong cross-section that circles the outside of the particle bed and borders the inside of the particle containing vessel. The outer screen element can also be provided by a cylindrical screen or basket structure that retains particles in its interior and provides a gas distribution space about its exterior. This invention does not apply to such cylindrical screen or basket arrangements. One familiar geometry for the oblong conduits has a scallop shaped cross-section and such conduits are hereinafter referred to as scallops. Scallops are preferred in many applications due to lower cost and simplicity of design compared to many continuous screen designs. The conduits have the oblong or scallop shape so that the backs of the conduits will fit closely against the wall of the vessel thereby minimizing the volume between the back of the conduit and the vessel and maximizing the central bed volume of the vessel.
A common type of scallop design now used in radial flow reactors is described in U.S. Pat. No. 5,366,704. This disclosure is directed toward a scallop fabricated from a single sheet of material that is rolled to the desired shape and welded along a joint that extends vertically when the scallop is oriented in its normal position within the radial flow reactor. The scallop is perforated to allow contact of generally radially flowing gas with catalyst particles contained within an annular catalyst retention space. One very desirable feature of this design, commonly referred to as a "perforated plate" or "punched plate" scallop, is its ability to undergo significant radial stresses associated with differences in thermal expansion coefficients between the scallop and the catalyst particles, reactor wall, and other reactor internal structures. Thermal expansion is a major consideration because radial flow reactors are overwhelmingly used in processes (e.g. catalytic reforming) requiring elevated temperatures. During transient processing conditions (e.g. startup, shutdown, and plant upsets), perforated plate scallops can generally yield to thermal radially directed stresses and distort from their original shape without a material change in performance. Without this flexibility, other internal reactor structures or even the catalyst particles may become significantly damaged.
Radial stresses imparted on the scallops also result from intermittent heating and cooling of the radial flow reactor when, for example, operation is non-continuous. When the reactor is heated for normal operation, it expands in both the axial and radial directions. Radial expansion permits an increased cross sectional area within the reactor vessel for catalyst particles comprising a catalyst bed to fill by gravity. When the reactor is later cooled for suspension of operation, the catalyst bed becomes compressed and thereby exerts a radially directed force on the scallops. Generally, catalyst particles have a considerably lower thermal expansion coefficient than reactor components and therefore change very little in dimension upon heating.
As is also understood in the art, particularly that pertaining to catalytic reforming using radial flow reactors, perforated plate scallops often lack the required rigidity in the longitudinal direction. As a result, these types of scallops have a tendency to collapse or buckle vertically under the combined radial and axial stress loadings exerted in normal operation. Axial stress, like radial stress, also stems mainly from thermal expansion, resulting in frictional resistance between the vertically "growing" scallop and catalyst particles in contact with the scallop outer face. The buckling phenomenon is perhaps best clarified using the example of a common, empty aluminum beverage can. A typical such container can support a significant load (e.g. the weight of an average person) in the axial direction. Nevertheless, when even a small radial force is applied to the side of the can (e.g. gentle pressure from a finger) in conjunction with this vertically directed stress, the can is easily crushed.
In the case of perforated plate scallops, as is recognized in the art, improving this buckling resistance is unfortunately not simply a matter of using a thicker metal sheet in fabrication. This is because, as the metal sheet thickness increases, the structural demands of so-called "punch and die" fabrication equipment capable of withstanding the scallop manufacturing process become prohibitive. Additionally, the necessary slotted perforations may become obstructed from migration of metal displaced during punching of the slots to adjacent slot areas. As a practical constraint, therefore, the thickness of the sheet metal used for perforated plate scallops cannot be greater than that of the slot openings. In turn, the slot width, to ensure the segregation of gaseous reactants and catalyst at the scallop interface, is governed by the minimum catalyst particle size, which is generally about 1.2 mm. Therefore, the use of 18-gage metal represents a practical maximum of metal thickness for scallop fabrication.
More recently, the problem of improving scallop axial strength to overcome buckling failure has been addressed using a curved section of profile wire to comprise a perforated scallop front side, joined to a flatter metal back side. Profile wire for this application is normally in a form known in the industry as a Johnson Screen (available from U.S. Filter Company, St. Paul, Minn.) or as a profile wire screen available from other suppliers worldwide. Profile wire screen provides a highly regular slotted surface that is resistant to blockage (plugging) from extended contact of the outer face of the profile wire section with catalyst particles. Furthermore, the rigidity of the profile wire section, regardless of slot opening, can be tailored according to the spacing and thickness of support bars used to join each individual, parallel extending, profile wire. Normally, the support bars are welded to each profile wire at the inner face of the profile wire screen, that is, the face not in direct contact with the catalyst particles. Support bars and profile wire are generally affixed perpendicularly to each other.
These newer scallops, hereinafter referred to as "conventional profile wire scallops" have been used commercially with some success in terms of resisting buckling in the axial or vertical direction. However, the desired radial flexibility of these prior art designs has been found unacceptably compromised. In other words, axial stiffness has been achieved, but only at the expense of a corresponding and undesired increase in resistance to radial stresses. A potential problem with such a design is that the use of scallops comprising profile wire, due to their lack of radial flexibility, can provide a crushing force between the catalyst and the inner screen or central conduit. Because catalyst particles are present between the scallops and central conduit, and the catalyst is normally essentially non-compressible, the axial load can be easily transmitted to the central conduit. Ultimately, the central conduit may become crushed from such stress loadings, resulting in a potential loss of catalyst containment. In general, maintenance and repair costs associated with conduit damage are significant, especially in cases relating to catalytic reforming operations where several radial flow reactors are in a vertically stacked arrangement. In these situations, a damaged reactor internal structure may require the dismantling of several reactors. Furthermore, severe damage to downstream equipment is expected in cases where catalyst containment is lost, allowing catalyst particles to exit the reactor with the outlet gas flow.
Overall, therefore, the prior art has failed to provide a scallop design that exhibits both adequate resistance to buckling in the presence of longitudinally directed stresses combined with good flexibility in the presence of radially directed stresses.